1. Field of the Invention
The present invention relates to instrumented sensors and to apparatus and methods for measuring or predicting stress and/or stress component(s) at an interface, such as a bond line, a joint, etc., of mated bodies.
2. Description of the Related Art
Various conventional means exist for joining two mechanical bodies together. By way of example, one or more pin members may be disposed through aligned and mating apertures of the mechanical bodies. Another example of a means for joining mechanical bodies together is adhesive, which may be applied along an adhesive bond line at the interface of the two mated bodies. Alternatively, adhesives may be used in combination with mechanical connectors, such as pin members.
Under operating conditions, a variety of forces act on the interface. For example, in the case of a bond line, these forces include normal forces acting perpendicular to a bond line and shear forces acting along the bond line.
Apparatuses and methods have been known for instrumenting a joint means to measure normal and shear stresses. For example, the use of various types of strain gauges to measure such forces are well known. Sensing devices are also known in which such strain gauges are used in combination with bridge circuitry such as a Wheatstone bridge.
The accurate measurement of shear forces exclusive of normal forces, however, has eluded effective measurement. Shear forces may be particularly insidious forces, especially along an adhesive bond line. It is often highly desirable to design a component or system so that shear forces and their detrimental effects can be minimized. The accurate measurement of shear forces often is particularly difficult, however, because the magnitude of the force can change, sometimes rapidly, over time and because shear forces are often accompanied by normal forces.
In the case of solid rocket motors, for example, there is an adhesive interface or bond line between the solid propellant grain and the insulated casing member. This adhesive interface is commonly referred to as a liner and is often made of polyurethane adhesives and the like. The liner functions to provide the bond between the propellant and casing insulation with adequate adhesive strength to ensure that the interfacial bond will be capable of withstanding all of the stresses to which the propulsion subsystem may be subjected during ignition, launch, maneuver, etc. The shear forces present during rocket motor operation, especially at launch, place great stress on the liner. Failure of the adhesive bond at the liner can lead to cracking or premature discharge of the solid propellant, thus compromising the rocket motor operation.
Preparation of adequate liner compositions and structures requires accurate modeling of shear loads experienced at the propellant-insulation interface. However, the accurate measurement of the shear loads in this environment has been difficult. Conventional shear sensors are sensitive to normal loads, temperature changes and other varying conditions, experienced during rocket motor operation. These outside influences can lead to inaccurate shear stress measurements.